The present invention relates to an arrayed waveguide grating to be used in the field of optical transmissions.
Recently, in the field of optical transmissions, as a method of significantly increasing transmission capacity, optical wavelength division multiplexing transmission has been successfully researched, developed, and made practicable. As an example of an optical transmission element for optical wavelength division multiplexing transmissions, there is an arrayed waveguide grating (AWG) as shown in FIG. 9. The arrayed waveguide grating is formed so that waveguide forming part 10 having a waveguide composition as shown in the same figure is provided on substrate 11. The composition of the waveguide is as follows.
That is, first slab waveguide 13 is connected to the exit side of one or more optical input waveguides 12 which are disposed in parallel. A plurality of arrayed waveguides 14 disposed in parallel are connected to the exit side of the first slab waveguide 13, and second slab waveguide 15 is connected to the exit sides of the plurality of arrayed waveguides 14. A plurality of optical output waveguides 16 disposed in parallel are connected to the exit side of the second slab waveguide 15. The arrayed waveguides 14 propagate light led out from the first slab waveguide 13, and are formed so as to have lengths which are different from each other.
The optical input waveguides 12 and optical output waveguides 14 are provided so as to accord to the number of, for example, signal light beams which have varying wavelengths to be demultiplexed by the arrayed waveguide grating. Normally, the arrayed waveguides 16 are provided by a large number, for example, 100. However, in this figure, for simplification of the figure, the number of the waveguides 12, 14, and 16 are simplified.
An optical fiber at the transmission side, for example, is connected to the optical input waveguides 12, whereby wavelength multiplexed light is led therein. Light which has passed through the optical input waveguides 12 and has been led into the first slab waveguide 13 is spread by the diffraction effect, made incident onto the plurality of arrayed waveguides 14, and propagated in the arrayed waveguides 14.
Light propagated in the arrayed waveguides 14 reaches the second slab waveguide 15, and furthermore, is condensed by the optical output waveguides 16 and outputted. Since the lengths of the arrayed waveguides 14 are different from each other, phase differences occur in the light beams after being propagated in the arrayed waveguides 14. In accordance with the phase differences, the wavefront of the converged light inclines, and in accordance with the angle of this inclination, the light condensation position is determined. Therefore, the condensation positions of the light beams with varying wavelengths are different from each other, and output waveguides 16 are formed at these positions, whereby light beams with varying wavelengths can be outputted from the different optical output waveguides 16 for each wavelength.
For example, as shown in this figure, if wavelength multiplexed light beams with wavelengths of xcex1, xcex2, xcex3, . . . xcexn (n is an integer of 2 or above) are inputted from one optical input waveguide 12, these light beams are expanded in the first slab waveguide 13. Then, the light beams reach the arrayed waveguides 14, pass through the second slab waveguide 15, and as mentioned above, are condensed on different positions at the exit end of the second slab waveguide 15 for each wavelength. Thereafter, the light beams which are different in wavelength from each other are made incident onto different optical output waveguides 16, pass through the respective optical output waveguides 16, and are outputted from the exit ends of the optical output waveguides 16. When an optical fiber for outputting light is connected to the exit ends of the optical output waveguides 16, the light beams with varying wavelengths are taken out via this optical fiber.
In the arrayed waveguide grating, the wavelength resolution of the grating is in proportion to the differences (xcex94L) in length between the arrayed waveguides 14 which comprise the grating. Therefore, by properly setting the xcex94L, wavelength multiplexed light can be demultiplexed at narrow wavelength intervals. As an example of such an arrayed waveguide grating, an arrayed waveguide grating is proposed which is arranged so that the difference in optical path lengths between the arrayed waveguides 14 is set to 65.2 xcexcm, the order of diffraction is set to 61, the total number of arrayed waveguides 14 is set to 100, and wavelength multiplexed light is demultiplexed into 32 waves at intervals of 100 GHz.
When manufacturing an arrayed waveguide grating, first, flame hydrolysis deposition is used to deposit and form a lower cladding layer on silicon substrate 11, and then the layer is consolidate. Next, flame hydrolysis deposition is used to deposit and form a core layer on the consolidate lower cladding layer, and then the core layer is consolidate. Thereafter, an arrayed waveguide grating pattern is transferred onto the core layer by means of photolithography and the reactive ion etching method via a photomask on which the arrayed waveguide grating is drawn.
Thereafter, the core is etched, and the waveguide composition (waveguide pattern) of the arrayed waveguide grating is formed. Thereafter, at the upper side of this waveguide composition, an upper cladding layer is formed by means of flame hydrolysis deposition and consolidate, whereby the arrayed waveguide grating is formed.
The number of deposited layers of the core is generally 6. In FIG. 10, an example of the transmission spectrum of the arrayed waveguide grating is shown. The arrayed waveguide grating having this transmission spectrum is manufactured so that the number of deposited layers of the core is 6, and the optical path length difference between the arrayed waveguides 14, the total number of the waveguides, and the order of diffraction are set to the abovementioned numerical values. In addition, regarding this transmission spectrum, the transmission bandwidth is standardized by the FSR (Free Spectral Range: 25 nm herein), and the transmittance is standardized by means of minimum loss. As can be clearly understood from this figure, in this prior-art arrayed waveguide grating manufactured as mentioned above, the isolation (hereinafter, referred to as cross talk) which is the gap between the transmission loss of the transmission wavelength (A of the figure) and the background transmission loss (B of the figure) is 30 dB.
In the abovementioned dense wavelength division multiplexing transmission system (hereinafter, referred to as the D-WDM transmission system), a crosstalk of approximately 40 dB is required for the arrayed waveguide grating to be applied to this system. However, in the abovementioned prior-art arrayed waveguide grating, since the cross talk is only 30 dB, the characteristics required from the D-WDM transmission system side cannot be satisfied.
Furthermore, the present inventor considers that the deterioration in the cross talk to a degree of 30 dB in the prior-art arrayed waveguide grating is caused by fluctuations in the propagation constant of the core comprising the arrayed waveguide 14. When the amount of deviation of propagated light within each arrayed waveguide from the equiphase wave surface is defined as a phase error, fluctuations in the propagation constant are the phase errors of the propagated light, which causes deterioration of the cross talk of the arrayed waveguide 14. That is, originally, light is condensed to a predetermined one point at the output end of the second slab waveguide 15 for each wavelength. However, if the light deviates due to the phase errors, the light is not condensed to the predetermined one point for each wavelength, but leaks to an adjacent channel, and the cross talk deteriorates.
Therefore, the present inventor calculated the numerical degree of influence of the phase error on the cross talk. The transmission spectrum T (xcex) is expressed by the sum of the complex electric field distributions of light exited from each arrayed waveguide 14, and this can be expressed as (Formula 1).                               T          ⁢                      (            λ            )                          =                              "LeftBracketingBar"                                          ∑                                  m                  -                  0                                                  M                  -                  1                                            ⁢                                                                    A                    m                                    ⁢                                      (                    λ                    )                                                  ⁢                                  ⅇ                                      j                    ⁢                                          xe2x80x83                                        ⁢                                                                  φ                        m                                            ⁢                                              (                        λ                        )                                                                                            ⁢                                  ⅇ                                      j                    ⁢                                          {                                                                                                    2                            ⁢                                                          xe2x80x83                                                        ⁢                            π                                                    λ                                                ⁢                                                                              n                            eff                                                    ⁢                                                      (                            λ                            )                                                                          ⁢                        m                        ⁢                                                  xe2x80x83                                                ⁢                        Δ                        ⁢                                                  xe2x80x83                                                ⁢                        L                                            }                                                                                            "RightBracketingBar"                    2                                    (                  Formula          ⁢                      xe2x80x83                    ⁢          1                )            
In (Formula 1), xcex is the wavelength, M is the number of arrayed waveguides, Am is the light amplitude of the light electric field distribution emitted from the m-th arrayed waveguide, and neff is the effective refractive index of the arrayed waveguide. xcex94L is the optical path length difference between the arrayed waveguides, j is an imaginary number (j=(xe2x88x921)), xcfx86m is the phase error between the arrayed waveguides, and this phase error is in accordance with the standard regular distribution of the standard deviation "sgr"(xcfx86). Furthermore, in the arrayed waveguide grating manufactured by means of the prior-art, the number M of arrayed waveguides is 100, and the optical path length difference xcex94L of the arrayed waveguides is 65.2 xcexcm.
FIG. 3 shows an example of the transmission spectra calculated based on the abovementioned (Formula 1). One (solid line) of the two transmission spectra shown in the figure is the result of calculation of the transmission spectrum in a case where no phase error occurs. Meanwhile, the other transmission spectrum (dashed line) in the same figure is the result of calculation of the transmission spectrum in a case where a standard deviation of "sgr"(xcfx86)=0.6 rad exists in the phase error distribution. The relationship between the cross talk and the standard deviation "sgr"(xcfx86) of the phase error distribution in the arrayed waveguide grating can be obtained from the results of calculation of the transmission spectra as mentioned above, and the results thereof are shown in FIG. 4. As can be clearly understood from FIG. 4, the cross talk deteriorates as the standard deviation of the phase error distribution increases. In addition, in the arrayed waveguide grating manufactured by means of the prior-art, since the standard deviation of the phase error distribution is approximately 0.85 rad, it is expected that the cross talk will be approximately 30 dB.
The present invention is made in order to solve problems in the prior-art based on the results of the examination mentioned above. The object of the invention is to provide an arrayed waveguide grating in which the cross talk can be prevented from deteriorating by reducing the phase error of light propagated in the arrayed waveguides.
In order to achieve the above object, the invention employs the following aspects as means for solving the problems. That is, according to the first aspect of the invention, an arrayed waveguide grating in which a plurality of optical signals with varying wavelengths which are inputted from optical input waveguides are propagated while the signals are provided with phase differences for each wavelength by arrayed waveguides, and made incident onto different optical output waveguides for each wavelength, and light beams with varying wavelengths are outputted from the different optical output waveguides, comprising:
one or more of the optical input waveguides disposed in parallel;
a first slab waveguide connected to the exit side of said optical input waveguides;
the plurality of arrayed waveguides, which are disposed in parallel, and have lengths different from each other to propagate light led out from said first slab waveguide, and are connected to the exit side of said first slab waveguide;
a second slab waveguide connected to the exit side of said plurality of arrayed waveguides; and
the plurality of optical output waveguides disposed in parallel and connected to the exit side of said second slab waveguide, wherein
the standard deviation of the phase error distribution occurring within the plurality of arrayed waveguides is suppressed to 0.6 rad or less.
Also, according to the second aspect of the invention, in addition to the abovementioned first aspect, the standard deviation of the degree of fluctuation in the refractive index of the core comprising the plurality of arrayed waveguides is made to be 4.84xc3x9710xe2x88x926 or less.
Also, according to the third aspect of the invention, in addition to the first or second aspect, the core comprising the arrayed waveguides is formed by means of flame hydrolysis deposition, and the number of deposited layers of said core is set to 13 or more.